diy compression results not sure what they mean
 

so i got this 2004 rx8 for 1000 because the guy was tired of dealing with it and thought the engine was blown. Before i sell it for a quick profit, i wanted to be sure it was low on compression. I didnt want to trailer it to a mazda dealer to spend more money so i pulled the schreder valve out of my compression tester and the pulled the bottom spark plug out of the first rotor. Tested it and watched the needle bounce while holding gas pedal to the floor and a brand new battery. Then i did the same on the other. And watched it.

I live about 800 ft above sea level and its about 65 outside.
The first rotor was showing 68-71 psi each side of the rotor. The second was showing 70-72psi. The current engine was replaced per recall and has about 40k miles.

The po put a $1200 exaust system and i think it was running super rich. The plugs were absolutely covered in crusty oil gunk. I cleaned them best i could. The car starts odd and will eventually idle. Here is the first post i made with pics

https://www.rx8club.com/showthread.php?p=4438031#post4438031

I know this isn't super accurate but doesn't this at least tell me that it might not be a blown engine and I should try to fix? Or is this so inaccurate I need to get to a mazda dealer? I was thinking take the exhaust off to check for a clogged cat next and change the plugs....

A compression test on a rotary engine requires a special compression tester. Having said that, using a piston engine compression tester is inaccurate (my understanding is that you used a piston engine compression tester). What do you mean by the car starts odd? Does it take a long time to start? How does it run? How is acceleration? Any CELs? How about maintenance history? Further, you know that the rx8 has a bad resale value. Unless you got the car for free there is no way you will make any money.

Car has good maintenance records and has been maintanined by mazda dealer. I did use a compression tester from my piston engine. I realize this isn't ideal but wouldn't this at least be positive compared to some that test like this and get 5psi on one rotor?

When I start the car and it is cold I have to crank for about 4 seconds then gas a little. Then the car will stall if I let go of the gas. It also is rough on idle and poor rev sound. However if it warms up and then I restart it will idle with a small blip every two seconds or so. It will rev smootly to 7k but I haven't taken it past there.

I haven't wanted to drive it so have not tested it past this. Other than moving it off my trailer and into the garage.

I get that this test is inaccurate but explain why it would be so consistent if the engine had lost compression and that I can't at least establish that either a: its not totally gone, or b: some how miraculously the rotors have each lost compression identically on all faces over two sections.

I am not disagreeing with you as far as that you can get an idea of the engine health using a piston engine compression tester. I am just saying that readings are a bit off because you get no info in all three faces of the rotors. Anyway, you might want to give this a try: Compression Test

Some members here have tried it...

i'm not sure what you're trying to find out, 70psi is low. since its evenly low, that points to the thing being worn out, vs having a broken seal.

for example, one bad apex seal, will affect 2 chambers, so compression will be something like 0-0-110psi.

since its even, it should start cold and then run ok, i would expect it to not restart hot though.

if it doesn't run ok, you should check spark. next step might be to do some kind of decarbon (either the zoom zoom, seas foam, or just water), if the engine doesn't run properly it won't have good compression.

When we ran a piston engine compression tester on my motor it showed around 110-115psi per rotor face. Just a comparison. We did it before we took it in and had the dealer perform a proper test.
Most dealers charge around $150ish to do a compression test and they will give you numbers based on a 1-10 scale. Mine were all in the upper 8's.

I believe they normalize it to a specific RPM. 100 is decent otherwise... 90 is likely borderline.

800foot elevation will have a noticeable affect

You'll need to compensate for that as well as the extra volume in the adapter that screws in to the spark plug hole.

65 degree's mmmmm

Didn't think about the adapter but that makes sense. I took the valve out of the compression tester so I could see the needle jump each time for each face and they all were right around 70psi. I'm starting to think carbon buildup is signifigant and I'm not sure if both rotors are working. I might try the seafoam and a set of new plugs next along with disconnecting the exhaust. It definately runs better when warmed up but not as it warms up. It gets better if I start it again after it warms up and takes no gas to get it started unlike when the engine is room temperature.

So to clean out the carbon and crap without disassembly would be seafoam when its warm? Should I do it multiple times to be sure and will the rotary be hurt by it? Just wondering if its different from piston engines.

Check out this thread for the different types of decarbing, and the impact each has: https://www.rx8club.com/tech-garage-...r-pics-241867/

^^^ You ever try ATF RIWWP?

No, I did not. I'm willing to do that test, but need a donor 8 first now :)

Mine is just sitting in the garage if you wanna come get it.

While you're at it ... weld up my cold and hot side plumbing too please
It looks like poop

Sure, thanks for paying for my welding classes and welding equipment. I'll be right up as soon as I graduate.

LOL no problem, just trying to help.
You know turn over a new leaf.

Can I interest you in a minor in cooking, perhaps?

Sorry OP for hijacking your thread. My fault, I'll stop now.

@ the OP
Where do you live?
Perhaps someone in your area is willing to do a compression test for you?

Some of us have reasonable equipment to do such things, others actually have the tester Mazda uses.
Most of the members here will do it for free.

If you live in the Toronto area, maybe we can get together.

You need to have rpm to correct your compression numbers. There is an android/iphone app that does the conversion, but you still need an obd2 reader to get rpm. 70psi may be fine if you are only turning at 200rpm.

Here is the actual copied calculator from the Mazda dealer network:
Foxed.ca - Rotary Compression Calculator

I live in sumter sc so close to columbia. Man, plugs are 20 bucks a piece. But seeing how absolutely terrible the ones I pulled were, I see no choice to really do any type of trouble shooting. So if I grab some seasfoam and run it through an entire bottle or two, should I start that process with new plugs or use the ones I have?. One of my plugs even had the inside core tip was cracked. That right there might be one side that wasn't firing completely and the others might just be so corroded I'm only firing 50% of the time....

I appriciate the help, I'd rather not drop more money toward it if its not going to pay off in the end. Right now I've seen 5 in same condition not running go for well over 2k. For me I wouldn't mind a quick 1k profit but I'd rather have an awesome running rx8 to go with my ITA rx7.....

the one time i did it, the fire dept showed up, it creates a lot of smoke. like the whole block is on fire.

water didn't smoke at all (i metered it in slowly), and if you've ever seen a piston engine with a blown head gasket, probably works just as well or better.

^ he was referring to if I tested ATF for the before and after. See the thread I linked, it has the actual before and after rotor pics of water, seafoam, and washer fluid, two different methods used to decarb. Shows what actually happens inside the engine.

I did not test ATF.

check for spark, i just use a timing light, although the HEI tester is probably better.

i recently bought a "dead" Rx8, the owner swore it was dead, and had a receipt from a shop that said it was dead.

turns out it needed coils, and plugs, and some deflooding.

motor is totally fine.

Use your old spark plugs to seafoam it....or else you might foul the news one.